1. Field of the Invention
The invention is concerned with magnetic "bubble" devices. In particular, the invention is concerned with devices which include a supported layer of magnetic garnet material, generally, but not necessarily, on a non-magnetic garnet substrate. Such devices depend for their operation on nucleation and/or propagation of small enclosed magnetic domains of polarization opposite to that of the immediately surrounding material in the supported layer. These domains have come to be known as "magnetic bubbles". Functions which may be performed include switching, memory and logic.
2. Description of the Prior Art
A magnetic bubble is a magnetic domain characterized by a single domain wall which closes upon itself in the plane of a layer of magnetic material in which it can be moved. Inasmuch as the wall closes on itself, the domain is self-defined and is free to move anywhere in the plane. Domains of this type are disclosed in U.S. Pat. No. 3,460,112 of A. H. Bobeck et al. issued Aug. 5, 1969.
A layer of magnetic material in which bubbles can be moved typically includes an epitaxially grown single crystal film having a preferred direction of magnetization normal to the plane of the film. A domain in such a material is visualized as a right circular cylinder magnetically positive at the top surface of the layer and negative at the bottom forming a magnetic dipole along an axis normal to the plane of movement. When exposed to polarized light and viewed through an analyzer, a single wall domain appears as disk relatively dark or light, in contrast to the remainder of the layer (thus, the term magnetic bubble).
When a suitable layer of magnetic material is maintained in a bias field perpendicular to the layer, a bubble is stable over a range of bias fields, which corresponds to a (stability) range of diameters. This range of diameters varies from a maximun at which a bubble "strips out" (at low bias field) to a finite minimum at which the bubble collapses (at high bias field), a range in which the maximum and minimum diameters differ by a factor of about three. The upper end of the corresponding bias field range is termed the "bubble collapse field" and the lower end of the range is termed the "strip out field". To ensure the widest possible operating margins in a practical bubble device, a bias field is typically chosen to produce a characteristic diameter in the middle of a bias range, which corresponds to the stability range of diameters.
In much of the literature, it has been prescribed by those skilled in the art, that a layer of material suitable for the movement of single wall domains be characterized by properties which ensure a stability range of magnetic field which is ideally constant as a function of temperature over a practical temperature range and that the bias field be maintained at a preselected constant value. Inasmuch as the stability range of a layer of selected material varied, the operating margins were reduced. The properties of magnetic materials and the relationship of those properties to the stability range are discussed in the Bell System Technical Journal, Vol. 50, No. 3, March, 1971, at page 725 et seq., in an article by A. A. Thiele, entitled "Device Implications of the Theory of Cylindrical Magnetic Domains".
A modification of this device design concept, which has lead to extended operating temperature ranges is disclosed in U.S. Pat. No. 3,711,841, issued Jan. 16, 1973. This patent discloses the utility of temperature varying materials, if a bias magnet structure is used which produces a temperature varying bias field to approximately match the temperature variation of the material. However, to extend the range of utility of bubble devices it would be desirable to tailor the bubble material properties to better match the temperature variation of available and otherwise desirable bias magnet materials. This has been done by controlled octahedral substitution through control of the concentrations of constituent oxides in the melt (U.S. Pat. No. 4,002,803, issued Jan. 11, 1977 to S. L. Blank) and by control of Ge substitution by selection of growth temperature (copending application Ser. No. 607,378; filed Aug. 25, 1975 now U.S. Pat. No. 4,034,358). However, the crystal growth systems employed are quite complex so that additional degrees of freedom are desired in order to tailor material properties to ever more stringent device requirements.